The present invention relates to test set apparatus for analyzing circuit characteristics. More particularly, it relates to phasor impedance measuring test set apparatus for use in making measurements on electrical circuits including cable transmission systems, particularly telephone communication cables.
Telephone cable and communication network circuit analysis, including performance evaluation, troubleshooting of existing installations and acceptance testing of new installations for faults, is greatly facilitated by having knowledge of the electrical impedance and return loss looking into the two-wire ports of the cable or network circuit. For example, testing of subscriber loop performance is facilitated by having knowledge of the impedance on subscriber loops looking out from the central office.
Traditionally, the telephone industry has utilized return loss measurements as the accepted characteristic to quantify the impact of impedance mismatches on the design and operation of communications circuits and networks. Return loss is a measure of the signal power that will be reflected at a point of impedance discontinuity along a transmission path. In effect, return loss in one measure of the efficiency of signal propagation at impedance mismatches. For example, if the return loss at a selected point along a transmission line is 10 dB, then one-tenth of the incident signal power that appears at that point will be reflected toward the source; that is, "returned" toward the originating point. Nine-tenths or 90% of the incident signal power will propagate beyond the point of impedance mismatch.
Return loss measurements, although related to impedance, do not give an indication of the actual impedance values occurring at the point under consideration. The telephone industry has adopted measurement of return loss in their standard methods primarily because of the lack of practical instrumentation to measure impedance directly. Existing apparatus for measuring impedance have been in the form of balanced bridges, such as the familiar Wheatstone bridge, or other similar apparatus, such as level tracer instruments. These types of apparatus have been too time-consuming to use and lacked the ability to measure impedance in the presence of unwanted signals that normally occur on practical telecommunications circuits.
Accordingly, there has developed a need for improved test set apparatus for measuring impedance, which is suitable for use in telephone cable and network circuit analysis testing.